A method of irradiating an object with radiation, detecting the intensity distribution of radiation having passed through the object, and obtaining the radiological image of the target has widely been used in industrial nondestructive inspection and medical diagnosis. Recently, a digital radiographic apparatus using a two-dimensional image sensing element is also developed, as disclosed in Japanese Patent Laid-Open Nos. 09-321267 and 09-073144.
In the digital radiographic apparatus using an image sensing element, the gains of pixels are not uniform, and gain correction for each pixel is necessary to obtain a proper output image from an image formed on the image sensing element. Gain correction image sensing is called calibration, which is generally executed by the user at a predetermined interval. In calibration, the entire effective image sensing region is irradiated with radiation in the absence of any object to be sensed. The sensed image (to be referred to as a “gain image”, hereinafter) is stored, and gain variations in a clinical image sensed for diagnosis are corrected using the gain image. In this case, irradiation nonuniformity of the radiation tube lamp and transmission nonuniformity of grid and the phototimer can also be corrected at the same time as correction of gain variations in pixels. A more accurate image than an analog radiograph can be acquired.
However, the gain distribution characteristic of the radiographic apparatus is not invariant. For example, when the radiation quality changes due to the difference in tube voltage, the shadow of the phototimer or grid may be sensed. The degree of variations between pixels may change due to the difference in the ambient temperature of the image sensing element, and the internal structure or driving structure of the radiographic apparatus may be seen. Shading by cut-off of the grid may be emphasized, or the edge of the phototimer may be slightly sensed owing to the difference in the incident angle of radiation. Such phenomenon degrades the suitability of an output image, and further generates a processing error in image processing which helps to diagnose an image, outputting an image not suitable for image diagnosis.
To prevent this, calibration is desirably performed after conditions for sensing an object image are determined. Calibration generally executes image sensing a plurality of number of times, which is cumbersome to the user. It is not practical to perform calibration for every image sensing. From this, generally, calibration is periodically executed and the latest gain image is used for correction.
A large difference between image sensing conditions in calibration image sensing and image sensing conditions in actual object image sensing may generate an artifact. The user is recommended to periodically perform calibration without determining whether calibration is necessary. As a result, an effective gain image may not be efficiently acquired.